An image forming device generally is commonly referred to as an image display device or a “projector,” and such projectors may include a cathode ray tube (CRT) projector, a liquid crystal projector, or a DMD (Digital Micro mirror Device) projector. A DMD is a reflection type image forming device or image display device. A DMD includes a plurality of micromirrors arranged in a two dimensional array that turns the reflected light on and off by changing a tilt angle of each individual micromirror.
A projector has been used for information sharing at office and school and watching movies and other images. Recently, it is used for digital signage that displays advertisements and guidance on electronic media and is also used in a head-up display (HUD) system which projects an image on a small screen arranged in a car. Such a head-up display system causes the driver to observe the image via a concave mirror and a windshield of the car.
However, a projector used for digital signage has a problem that an image projected on a projected surface becomes trapezoidal and a focus of the projector becomes unbalanced at the top and bottom of the image when the projected surface is inclined with respect to the optical axis in the case of oblique projection. When focusing on the bottom edge of the image, the top edge of the image will not be in focus, so a method to reduce the focus imbalance by focusing on the center of the image is used.
In a projector for an automobile HUD, it is sometimes preferable that the image projected on the small screen has a trapezoidal shape. The image to be projected is, for example, a speedometer, a road sign, or the like, and the unbalance of focus of the image is required to be minimal.
As a method for eliminating the imbalance in focus of the image, a method of adjusting the optical system to satisfy the Scheimpflug condition may be used. For example, by tilting the image plane (image forming element) in accordance with the inclination of the object plane (projected plane), it is possible to focus on the entire area to be projected. The amount of tilting the image forming element is proportional to the magnification of the lens system.
On the other hand, in the design of the illumination optical system of the general projector, it is required to uniformly illuminate the image forming element. For example, an optical system that illuminates an image forming device using a light tunnel, and shows a configuration of an illumination optical system for forming a real image at the exit of the light tunnel on the image forming device.
A configuration using a fly's eye lens may also be used. A system in which light from a light source is once divided into a plurality of light fluxes by a fly's eye lens and is converted into a single bundle of rays again on the image forming element, thereby achieves uniform illuminance. Also in this case, the lens between the fly's eye lens and the image forming element is designed so as to be in focus on the entire image forming surface of the image forming element, and the image forming element is uniformly illuminated.
As a light mixing element, the method of making the light amount uniform is different between a light tunnel and a fly-array lens, but in either case, there is a condensing optical system composed of a lens or a mirror between the light mixing element and the image forming element. As described above, in a general projector, it is proposed that a technique of projecting an image having a uniform illuminance distribution onto the projected surface by making the illuminance distribution on the image forming element uniform.
However, in a case where the projected surface is inclined with respect to the optical axis of the projection optical system and the image to be intentionally projected is trapezoidal (for example, digital signage or projection in a HUD system for automobiles), if the illuminance on the image forming element is uniform, the illuminance distribution of the projected image becomes unbalanced.
Since the illuminance is a value obtained by dividing the light quantity by the area, the illuminance on the short side of the trapezoidal shape image is high and the illuminance on the long side is low, so that for the observer, the short side is bright and the long side It will be visually recognized as a dark unbalanced image.
Accordingly, an object of the present application is to provide an image display device capable of eliminating an imbalance in illuminance distribution caused by a trapezoidal shape when the projected image is a trapezoidal shape.